1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a polishing apparatus having an exhaust system for exhausting an interior of the polishing apparatus. The polishing apparatus may include a polishing section for polishing a surface of the workpiece and a cleaning section for cleaning the workpiece which has been polished.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical polishing.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains (or material) is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated.
There has been proposed such a polishing apparatus which incorporates a polishing section for polishing semiconductor wafers and a cleaning section for cleaning the polished semiconductor wafer in an enclosing structure such as a housing. To be more specific, the polishing apparatus comprises an enclosing structure, a polishing section for polishing semiconductor wafers, a cleaning section for cleaning the semiconductor wafers which are to be polished or have been polished, and a transfer robot (or robots) for transferring the semiconductor wafers which are to be polished or have been polished.
When a wafer cassette containing a plurality of semiconductor wafers to be polished is set in a predetermined position in the polishing apparatus, a hand of the transfer robot takes out a semiconductor wafer from the wafer cassette and transfers the semiconductor wafer to a transfer unit in the polishing section. The semiconductor wafer is then transferred from the transfer unit to a top ring, and is held by the top ring under vacuum. The top ring is moved above a turntable, and the rotating top ring presses the semiconductor wafer against a polishing surface of the rotating turntable, whereby the semiconductor wafer is polished to a flat mirror finish while an abrasive liquid is supplied to the polishing surface of the turntable.
After the semiconductor wafer is polished, it is returned to the transfer unit, and then transferred to the cleaning section by the hand of the transfer robot. In the cleaning section, the polished semiconductor wafer is cleaned to cause abrasive material adhering to a surface of the semiconductor wafer to be removed, and thus a cleaned surface of the semiconductor wafer is obtained. After the semiconductor wafer is cleaned by the cleaning section, it is transferred to a spinning drier by the transfer robot, and then spin-dried in the spinning drier. The cleaned and dried semiconductor wafer is returned to the wafer cassette by the transfer robot.
In the polishing section, during polishing, mist of the abrasive liquid is scattered around, and therefore the mist of the abrasive liquid or ground-off particles generated in the polishing section are exhausted from the interior of the polishing section to the exterior of a clean room in which the polishing apparatus is installed. Similarly, in the cleaning section, since organic solvent or acid cleaning solution is evaporated, ambient air in the cleaning section is exhausted from the interior of the cleaning section to the exterior of the clean room so that ambient air in the polishing apparatus is prevented from flowing into the clean room having a clean ambient air environment.
In the polishing apparatus having the above structure, a door is provided for carrying in or carrying out the wafer cassette, monitoring an operational condition, or performing maintenance of various devices. It is therefore necessary to exhaust ambient air in the interior of the polishing apparatus by an exhaust system having a duct and an exhaust fan so that mist or particles in the polishing apparatus are not discharged to the clean room when the door is opened. As an exhaust condition, an exhaust speed of 0.4 m/sec or more is maintained when the door is opened.
However, since an exhaust operation of the exhaust system is fixed, i.e., is unchanged even when the door is closed, the exhaust capability (or exhaust capacity) of the exhaust system is constant. That is, the exhaust system exhausts more air than necessary. When the door is closed, if negative pressure relative to the clean room environment is developed in the polishing apparatus, this state is sufficient to prevent ambient air in the polishing apparatus from being discharged to the interior of the clean room. However, since the exhaust system is always operated at the same exhaust capability, the operating cost of the exhaust system increases.
Further, clean air supplied from the clean room to the polishing apparatus is discharged to the exterior of the plant by the exhaust system. Since clean air used in the clean room is rather expensive, if the amount of clean air discharged from the polishing apparatus is reduced, the operating cost of the whole plant can be reduced because the amount of clean air supplied to the clean room is reduced.